Satellites are used in many aspects of modern life, including earth observation and reconnaissance, telecommunications, navigation (e.g., global positions systems, or “GPS”), environmental measurements and monitoring and many other functions. A key advantage of satellites is that they remain in orbit due to their high velocity that creates an outward centripetal force equal to gravity's inward force. Therefore, once in orbit, they stay there typically for years or decades. See, for example, FIG. 8, which graphically illustrates a best and worst case curve for expected lifetime of orbiting vehicles as a function of altitude. Depending on the angle of the orbit, a satellite will be able to observe a large fraction of the earth's surface at some point in time.
A key parameter for satellites used for earth observation is the relationship between altitude, orbital angle, and constellation size. At higher altitudes, the satellite will be able to observe a larger percentage of the earth's surface, however the orbital time will be longer and the instrument package required to effectively cover a larger area at a longer range will be larger and more complex, on the other hand, a longer orbital time means that the satellite will appear to be in view of a given point on the earth for a longer period and the number of satellites required to keep all of the earth in view all of the time decreases. In order for one satellite to cover the entire surface of the earth, sun synchronous polar orbits are frequently used.
Satellite orbital heights are typically categorized in three broad segments: low earth orbit (LEO), medium earth orbit (MEO) and geostationary earth orbit (GEO). The general uses and characteristics of these orbits are shown in Table I and represent generally accepted usage of the terms LEO, MEO and GEO. Satellites can orbit at any altitude above the atmosphere, and the gaps in altitude shown in Table 1, such as between LEO and MEO, are also used, if less regularly. It is also common that satellites may orbit in eccentric, non-circular orbits, thereby passing through a range of altitudes in a given orbit.
TABLE ITypical characteristics of common orbits.LEO  400-6.9-7.8Earth observation,Random orbits, 3-10 Y 2,000sensing, ISS, lifetime, space junk telecomissue, little radiationconstellationsMEO15,000-3.5GPS, GLONASS,Highest radiation (Van20,000Earth observationAllen Belt), equatorial topolar orbitsGEO42,0003.1Sat TV, high BWCan remain above sametelecom, weatherspot on Earth, typicallysatellitesequatorial orbits
For imaging, the power requirements of the digital optical package and downlink grows roughly in accordance with a square law for the same delivered image resolution. GEO satellites are far too high for a practical optical observation package. LEO, on the other hand, allows for reasonable optical size and power and is protected from space radiation. Most earth imaging satellites operate at lower altitudes, roughly at the altitude of the international space station (ISS) (400 km) or higher, up to about 2,000 km. At these altitudes, the size and power requirements of the imaging package are much lower for the same resolution relative to a geostationary orbit, the earth's magnetic field shields the satellite from most damaging space radiation, and the atmosphere is sufficiently thin that orbital decay is not a major problem. However, the satellite will only be in view of a given section of the earth's surface for a few minutes, and at lower altitudes, line of sight communication may only be possible for a minute or less. This requires a large constellation satellites or accepting a lower “revisit” rate for a given point on the Earth's surface.
Altitudes lower than the international space station (ISS) have the advantage that the imaging package can again, be substantially reduced in size, weight, and power consumption, which in theory allows for much lower cost satellites. However, atmospheric drag becomes a major consideration for orbits below the orbit of the ISS—even the ISS requires regular “boosting” to keep its orbit from decaying rapidly, and orbital decay issues grow exponentially below ISS altitudes. The assignee of the present invention has addressed this issue with a novel low drag orbital vehicle and constellation design described in co-pending application Ser.No. 15/868,794, entitled “System For Producing Remote Sensing Data From Near Earth Orbit,” to Thomas E. Schwartzentruber and Ronald E. Reedy, and co-pending application Ser. No. 62/616,325, entitled “Atomic Oxygen-Resistant, ZLow Drag Coatings And Materials,” to Timothy Minton and Thomas E. Schwartzentruber. This enables Near Earth Orbiters, NEOs, a term we use to describe the system and its constituent vehicles (i.e., a “NEO satellite system”, “NEO vehicle” or a “NEO satellite”) operating in stable orbits at 100-350 km. Therefore, it is a purpose of this invention to describe a satellite system based on orbital vehicles operating in stable Earth orbits at altitudes well below traditional satellites, specifically between approximately 100 and 300 km.
The availability of a low drag, low cost, high endurance (multi-year missions) satellite suitable for altitudes under 300 km allows for the use of imaging equipment that is low cost and low power yet achieves resolutions that are currently only available from much more expensive, higher altitude satellites and much higher revisit rates because the constellation can be large for the same cost. There remains, however, the problem of retrieving the high revisit rate, high-resolution data collected by a large constellation of NEO satellites. Smaller constellations at higher orbits solve this problem either because the downlinks are relatively lower data rates per downlink (voice calls), or using very high bandwidth, but expensive earth stations. Higher orbits also imply long revisit times—for a single satellite, the revisit time may be measured in days. At sub 300 km orbits, the orbital window for a downlink to a given station is too small to tolerate a small number of earth stations with the power available in a small satellite. Furthermore, in order to fully take advantage of the high revisit rates possible in a large NEO constellation, the downlink must be sufficiently robust to allow for near real time download of data, with latency on the order of seconds or at most, a few minutes. In the event that a downlink is not immediately available, the processor can buffer the captured data for later transmission when a receiving station is within range. Consequently, the ground station network must in some respects mirror the satellite network, with a large number of ground stations to ensure that a given satellite can be continuously pushing its image data down. Achieving low power consumption for a given bandwidth is also essential, since the small satellite profile required to achieve a low per satellite cost has a correspondingly small surface area available for solar panels to generate power for the downlink and antennas to provide the antenna gain.
The combination of these factors means that there is a need for a low cost, low power, high data rate satellite downlink system that meets the mission payload requirements for a small orbital vehicle under 300 km that still has a camera resolution on the order of 1 meter, coupled with a corresponding robust network of ground stations to match the capacity of the satellites and reduce the delay time in retrieving a high resolution image of a given point on the earth to sub one hour times.